Attention deficit hyperactivity disorder is one of the most common childhood disorders, and is thought to affect more children with intellectual disabilities than those without. It is associated with marked social and educational impairment, and is recognised as a major risk factor for the development of problems throughout adolescence and adulthood such as conduct disorder, educational failure and personality disorder. Although prevalence rates vary, current research suggests that attention deficit/hyperactivity disorder affects between 2% and 5% of typically developing children and up 16% of children with intellectual disabilities. However, there has been little exploration into why this population appear to be at increased risk. This study aimed to examine whether parental reports of hyperactivity were measuring the presence of an underlying liability to AD/HD in their children with intellectual disability, or whether they were being influenced by other factors, such as levels of adaptive behaviour or the presence of other behavioural problems. Parental reports of hyperactivity on a screening questionnaire were compared with semistructured clinical interviews and an objective measure ofAD/HD. Results found that the majority of children, who were reported by parents as being hyperactive on the screening measure, were also found to exhibit AD/HD symptoms on subsequent measures. These results support findings from previous studies that report AD/HD as being commonly exhibited by children with intellectual disability. However, they also highlight a need for more research to be undertaken on the specificity of AD/HD in children with intellectual disability. This would allow advancements to be made in the domains of assessment, diagnosis and intervention for children with intellectual disability and AD/HD.